TITLE: GCN CIRCULAR NUMBER: 15145 SUBJECT: Konus-Wind observation of GRB 130831A DATE: 13/08/31 18:50:31 GMT FROM: Dmitry Frederiks at Ioffe Institute S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf of the Konus-Wind team, report: The long-duration soft-spectrum GRB 130831A (Swift-BAT trigger 568849: Hagen, et al., GCN 15139) triggered Konus-Wind at T0=47062.044 s UT (13:04:22.044) The light curve shows a smooth pulse peaked at ~T0+1.6 s followed by a weaker emission till ~T0+35~s. The emission is seen up to ~6 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB130831_T47062/ As observed by Konus-Wind, the burst had a fluence of (7.6 ± 0.4)x10-6 erg/cm2, and a 64-ms peak flux, measured from T0+1.600 s, of (2.5 ± 0.3)x10-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum (measured from T0 to T0+24.832 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -1.61 ± 0.06, the high energy photon index beta = -3.3 ± 0.3, the peak energy Ep = 55 ± 4 keV, chi2 = 118/97 dof. The spectrum at the maximum count rate (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -1.51 ± 0.10, the high energy photon index beta = -2.8 ± 0.1, the peak energy Ep = 67 ± 4 keV, chi2 = 91/96 dof. Assuming Gemini-North redshift z=0.4791 (Cucchiara & Perley, GCN 15144), and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.27, and Omega_Lambda = 0.73, we estimate the following rest-frame parameters of GRB 130831A: the isotropic energy release E_iso is (4.6 ± 0.2)x10^51 erg, the peak luminosity (L_iso)_max is (2.7 ± 0.3)x10^51 erg/s, and the rest-frame peak energy Ep,i = (81 ± 6) keV All the quoted results are preliminary.